CVE-2025-67725: Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can bloc...

Description

Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS). Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS Probability

0.40%

31.4th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-400

Affected Software

Vendor	Product	Versions
Tornadoweb	Tornado	< 6.5.3

References

Timeline

Published: Dec 12, 2025
Last Modified: Jun 17, 2026
Status: Analyzed

Frequently Asked Questions

What is CVE-2025-67725?

Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS). Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3.

How severe is CVE-2025-67725?

CVE-2025-67725 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 0.40% probability of exploitation in the next 30 days.

How do I fix CVE-2025-67725?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.